Free-space transmission dielectric properties measurement based on six-port technology

Authors: JULRAT, SAKOL - Oak Ridge Institute For Science And Education (ORISE); Trabelsi, Samir

Submitted to: IEEE Transactions on Instrumentation and Measurement
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/11/2021
Publication Date: 5/21/2021
Citation: Julrat, S., Trabelsi, S. 2021. Free-space transmission dielectric properties measurement based on six-port technology. IEEE Transactions on Instrumentation and Measurement. Volume 70 Pages 1-7.

Interpretive Summary: Modern agriculture require new fast and nondestructive tools for assessing quality attributes of agricultural products. Microwave sensing technology has shown potential for providing such tools. It has the advantage of being nondestructive, does not require contact with the material, and provide the desired quality attributes in real time. In recent year, a lot of progress has been made in our laboratory in terms of developing microwave sensors for determining moisture content, bulk density, and other quality attributes. These sensors can be used on line, on combine harvesters, or as bench-top instrumentation for routine measurements on grains, seeds, and nuts. However, to move this technology from the laboratory to field use new inexpensive and yet reliable microwave circuits are needed. This paper presents a breakthrough six-port transmission microwave circuit for measurements on low-loss materials such as grains, seeds, and nuts. For purpose of validation, this novel microwave circuit was used for measurements on dielectric slabs, and spheres of known dielectric properties. Results obtained from measurements at 5.8 Gigahertz compared very well to those performed with expensive commercial vector network analyzer. The new six-port transmission microwave circuit is inexpensive and suitable for implementation in portable microwave sensors for in-the-field use and can be easily integrated in sensing systems for on line monitoring and control without disturbing existing processes.

Technical Abstract: A free-space transmission technique for low-loss material dielectric properties measurement based on six-port technology is presented in this paper. The transmission coefficient is obtained by using a ratio of incident waves at port 1 and port 2 of the six-port network, while the power readings are performed at ports 3 to 6 simultaneously. The incident wave at port 1 comes from a forward propagating electromagnetic wave passing through the material sample. The incident wave at port 2 is a reference signal related to the incident wave at port 1 before or after propagating through the material. The calibration function is developed. The proposed six-port measurement technique is demonstrated at 5.8 GHz for purpose of verification. The material sample is placed between transmitting and receiving antennas. The microwave signal from the oscillator is fed to the transmitting antenna. The receiving antenna collects the signal and sends it to port 1. Two configurations are set up at port 2, connected to the microwave signal from the oscillator or terminated with an electrical load. Magnitude and phase measurement results for various thicknesses of dielectric slabs are comparable with those obtained with a commercial automatic vector network analyzer (ANA). Dielectric properties measurements on polypropylene, acrylic and acetal spheres also agreed well with those obtained by the ANA.